The present invention relates to a control system for controlling an engagement pressure of a start clutch of an automatic transmission which is engaged in starting a vehicle, and more specifically to a control system for controlling a hydraulic pressure to engage the start clutch which is disengaged to prevent creep of the vehicle when the vehicle is in a standing position and engaged when the vehicle is started.
A vehicle equipped with an automatic transmission tends to creep because of a drag torque of a torque converter when the selector of the automatic transmission is held in a forward drive position (especially of a high gear ratio) while the vehicle is at rest.
A Japanese patent provisional publication No. 59-13156 discloses a conventional system for preventing a creep of a vehicle. In this system, an electromagnetic valve is disposed in a drain passage which branches off from a supply passage for supplying a fluid pressure to a start clutch or the like of an automatic transmission, and which leads to an oil tank. The electromagnetic valve closes the drain passage when it is deenergized, and opens the drain passage when it is energized. The control system of this conventional example is arranged to prevent shock due to engagement of the start clutch in starting a vehicle from a standing position, by chopping a circuit for energizing the electromagnetic valve a predetermined number of times.
However, in this system, an engagement of the start clutch is retarded by reason of a lag of the hydraulic system and the inertial mass of clutch plates of the start clutch, so that the engine races and the engine speed rises abnormally.